Calcium efflux from internally dialyzed squid axons: the influence of external and internal cations

Overactivation of the sodium-calcium exchanger and transient receptor potential in anesthesia-induced malignant hyperthermia

Malignant hyperthermia is a pharmacogenetic disorder, which is an uncommon but frequently fatal intricacy of inhalation anesthesia in man. It causes a quick rise in body temperature to highly irreversible levels, which causes death in around three of four cases. The trigger anesthetics cause an anomalous, continued ascent in myoplasmic calcium levels. Possible mechanisms by which continuous release of sodium, calcium from skeletal muscle plasma membrane and sarcoplasmic reticulum stores respectively can produce the profound hyperthermia are discussed.

The kinetics of Ca-Na exchange in excitable tissue

A model is proposed to describe the Na-Ca exchange in excitable tissues. The present scheme requires a carrier mechanism that exchanges 3Na for 1Ca across the membrane under the electrochemical gradient of Na. The carriers, assumed to be trivalent anions, have monovalent and divalent sites; Ca and Na can compete only at the second site. The partially and fully loaded carrier-ion complexes are mobile and diffusible across the membrane. Subsequently, analytical expressions for Na and Ca unidirectional flux at steady state are derived in terms of intracellular concentration (Na(i) and Ca(i)) and extracellular concentration (Na(o) and Ca(o)) as well as membrane potential, E(M). Published experimental flux data on cardiac muscle, squid axon, and rat synaptosomes can be satisfactorily fitted with the flux equation simply by adjusting the numerical constants.

The sodium-calcium exchange system

Sodium-calcium counter-transport represents one of a number of processes for transporting calcium ions across cellular membranes. The physiological importance of the exchanger is outlined and its underlying mechanism discussed in terms of a comparison of the partial reactions of Na+-Ca2+ exchange in intact cells with those of plasma membrane vesicles.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions

The Na(+)/Ca(2+) exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca(2+) from the cell. Two basic properties characterize them. 1) Their activity is not predicted by thermodynamic parameters of classical electrogenic countertransporters (dependence on ionic gradients and membrane potential), but is markedly regulated by transported (Na(+) and Ca(2+)) and nontransported ionic species (protons and other monovalent cations). These modulations take place at specific sites in the exchanger protein located at extra-, intra-, and transmembrane protein domains. 2) Exchange activity is also regulated by the metabolic state of the cell. The mammalian and invertebrate preparations share MgATP in that role; the squid has an additional compound, phosphoarginine. This review emphasizes the interrelationships between ionic and metabolic modulations of Na(+)/Ca(2+) exchange, focusing mainly in two preparations where most of the studies have been carried out: the mammalian heart and the squid giant axon. A surprising fact that emerges when comparing the MgATP-related pathways in these two systems is that although they are different (phosphatidylinositol bisphosphate in the cardiac and a soluble cytosolic regulatory protein in the squid), their final target effects are essentially similar: Na(+)-Ca(2+)-H(+) interactions with the exchanger. A model integrating both ionic and metabolic interactions in the regulation of the exchanger is discussed in detail as well as its relevance in cellular Ca(i)(2+) homeostasis.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

The Na,K-ATPase hypothesis for bipolar illness

A cellular model for bipolar illness is presented. It is propounded that alterations in the activity of the membrane sodium- and potassium-activated adenosine triphosphatase pump (Na,K-ATPase) may be responsible for alterations in neuronal excitability and activity. Specifically, a reduction in Na,K-ATPase activity can lead to both mania and depression by increasing membrane excitability and decreasing neurotransmitter release, respectively. Supporting evidence is reviewed, and clinical and research implications are discussed.

Sodium-calcium exchange: derivation of a state diagram and rate constants from experimental data

A mechanism is developed for Na(+)-Ca2+ exchange using a new approach made possible by the availability of computer software that allows the systematic search of a large parameter space for optimum sets of parameters to fit multiple sets of experimental data. The approach was to make the experimental data dictate the form of the mechanism: the qualitative features of the data dictating the number and nature of the states of the exchanger and their interrelationship, and the quantitative aspects of the data dictating the values of the rate constants that govern the amount of each state relative to the total amount of exchanger. A single set of experimental data served this initial purpose, namely, observations of equilibrium Ca(2+)-Ca2+ exchange in cardiac sarcolemmal vesicles (Slaughter et al., 1983, J. biol. Chem. 258, 3183-3190). From this data a minimum mechanism was induced having 56 states (SYM56), which gave satisfactory quantitative fits to the experimental data. With this set of parameters additional experimental data were fitted, from the same preparation, the single cardiac cell and the squid giant axon, with some changes in parameters, but none dramatic. In spite of the symmetric nature of the mechanism, i.e. binding constants for Na+ and Ca2+ do not depend on the orientation of the binding sites, the mechanism exhibits marked asymmetric behavior similar to that observed experimentally. Finally, in accounting for Ca(2+)-Ca2+ exchange in the absence of monovalent cations, Ca2+ influx becomes dependent on intracellular Ca(2+)--an unexpected outcome--exactly in keeping with the "essential activator" role of intracellular Ca2+ observed by DiPolo & Beaugé (1987, J. gen. Physiol. 90, 505-525). Observations of Na(+)-Ca2+ exchange in the retinal rod outer segment are well fitted with a simplified version of SYM56 comprising 25 states (namely, SYM25), supporting the notion that the exchanger in the retinal rod outer segment differs from that in cardiac sarcolemma and squid axon. Maximum turnover rate of 840 sec-1 for SYM56 and 20 sec-1 for SYM25 are comparable to those reported for the exchanger in cardiac muscle and retinal rod outer segment, respectively.

The effects of manganese and changes in internal calcium on Na-Ca exchange fluxes in the intact squid giant axon

The effects of manganese chloride were studied on Na-Ca exchange fluxes from intact squid axons. Ca uptakes and Cao-dependent sodium efflux were inhibited half-maximally by 3-7 mM MnCl2. Mn inhibition appears less during Nao-Cai exchange (half-maximal inhibition; 30 mM) than that during Cao-Nai exchange, even when both fluxes were activated with 100 mM Na. The effects of changes in [Ca2+i], effected by Ca-EGTA injection or inhibition of mitochondrial Ca uptake by ruthenium red, were examined on the reverse (Cao-Nai) exchange mode. Ca-EGTA mixtures, designed to raise [Ca2+i] above 2 microM, inhibited Cao-Nai exchange fluxes. Ruthenium red inhibited mitochondrial Ca buffering to effect increases in Cai in the absence of Ca chelators; it activated Nao-Cai exchange fluxes but had little effect on Cao-Nai exchange despite similar reported Km for Cai. The results reflect the difficulty in demonstrating the stimulatory effect of [Ca2+i] on Cao-Nai exchange fluxes in intact axons.

Asymmetrical properties of the Na-Ca exchanger in voltage-clamped, internally dialyzed squid axons under symmetrical ionic conditions

In this work we have investigated whether the asymmetrical properties of the Na/Ca exchange process found in intact preparations are intrinsic to the exchange protein(s) or the result of the asymmetric ionic environment normally prevailing in living cells. The activation of the Na/Ca exchanger by Ca2+ ions, monovalent cations, ATP gamma S and the effect of membrane potential on the different operational modes of the exchanger (Nao/Cai, Cao/Nai, Cao/Cai, and Nao/Nai) was studied in voltage-clamped squid giant axons externally perfused and internally dialyzed with symmetrical ionic solutions. Under these conditions: (a) Ca ions activate with higher affinity from the inside (K1/2 = 22 microM) than from the outside (K1/2 = 300 microM); (b) experiments measuring the Cao-dependent Ca efflux in the conditions Lio-Trisi, Lio-Lii, Triso-Trisi, and Triso-Lii, show that the activating monovalent cation site on the exchanger faces the external surface; (c) ATP gamma S activates the Cao-dependent Ca efflux (Cao/Cai exchange) only at nonsaturating [Ca2+]i. Its effect appears to be on the Ca transport site since no alteration in the apparent affinity of the activating monovalent cation site was observed. The above results show that the Na/Ca exchange process is indeed a highly asymmetric transport mechanism. Finally, the voltage dependence of the components of the different exchange modes was measured over the range of +20 to -40 mV. The voltage dependence (approximately 26% change/25 mV) was found to be similar for all modes of operation of the exchanger except Nao/Nai exchange, which was found to be voltage insensitive. The sensitivity of the Cao/Cai exchange to voltage was found to be the same in the presence and in the complete absence of monovalent cations. This finding does not support the proposition that the voltage sensitivity of the Cao/Cao exchange is induced by the binding and transport of an external monovalent cation.

Kinetics and stoichiometry of coupled Na efflux and Ca influx (Na/Ca exchange) in barnacle muscle cells

Coupled Na+ exit/Ca2+ entry (Na/Ca exchange operating in the Ca2+ influx mode) was studied in giant barnacle muscle cells by measuring 22Na+ efflux and 45Ca2+ influx in internally perfused, ATP-fueled cells in which the Na+ pump was poisoned by 0.1 mM ouabain. Internal free Ca2+, [Ca2+]i, was controlled with a Ca-EGTA buffering system containing 8 mM EGTA and varying amounts of Ca2+. Ca2+ sequestration in internal stores was inhibited with caffeine and a mitochondrial uncoupler (FCCP). To maximize conditions for Ca2+ influx mode Na/Ca exchange, and to eliminate tracer Na/Na exchange, all of the external Na+ in the standard Na+ sea water (NaSW) was replaced by Tris or Li+ (Tris-SW or LiSW, respectively). In both Na-free solutions an external Ca2+ (Cao)-dependent Na+ efflux was observed when [Ca2+]i was increased above 10(-8) M; this efflux was half-maximally activated by [Ca2+]i = 0.3 microM (LiSW) to 0.7 microM (Tris-SW). The Cao-dependent Na+ efflux was half-maximally activated by [Ca2+]o = 2.0 mM in LiSW and 7.2 mM in Tris-SW; at saturating [Ca2+]o, [Ca2+]i, and [Na+]i the maximal (calculated) Cao-dependent Na+ efflux was approximately 75 pmol#cm2.s. This efflux was inhibited by external Na+ and La3+ with IC50's of approximately 125 and 0.4 mM, respectively. A Nai-dependent Ca2+ influx was also observed in Tris-SW. This Ca2+ influx also required [Ca2+]i greater than 10(-8) M. Internal Ca2+ activated a Nai-independent Ca2+ influx from LiSW (tracer Ca/Ca exchange), but in Tris-SW virtually all of the Cai-activated Ca2+ influx was Nai-dependent (Na/Ca exchange). Half-maximal activation was observed with [Na+]i = 30 mM. The fact that internal Ca2+ activates both a Cao-dependent Na+ efflux and a Nai-dependent Ca2+ influx in Tris-SW implies that these two fluxes are coupled; the activating (intracellular) Ca2+ does not appear to be transported by the exchanger. The maximal (calculated) Nai-dependent Ca2+ influx was -25 pmol/cm2.s. At various [Na+]i between 6 and 106 mM, the ratio of the Cao-dependent Na+ efflux to the Nai-dependent Ca2+ influx was 2.8-3.2:1 (mean = 3.1:1); this directly demonstrates that the stoichiometry (coupling ratio) of the Na/Ca exchange is 3:1. These observations on the coupling ratio and kinetics of the Na/Ca exchanger imply that in resting cells the exchanger turns over at a low rate because of the low [Ca2+]i; much of the Ca2+ extrusion at rest (approximately 1 pmol/cm2.s) is thus mediated by an ATP-driven Ca2+ pump.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium-calcium exchange current. Dependence on internal Ca and Na and competitive binding of external Na and Ca

Na-Ca exchange current was measured at various concentrations of internal Na [( Na]i) and Ca [( Ca]i) using intracellular perfusion technique and whole-cell voltage clamp in single cardiac ventricular cells of guinea pig. Internal Ca has an activating effect on Nai-Cao exchange beginning at approximately 10 nM and saturating at approximately 50 nM with a half maximum [Ca]i (Km[Ca]i) of 22 nM (Hill coefficient, 3.7). Measurement of Nai-Cao exchange current at various concentration of [Na]i revealed an apparent Km[Na]i of 20.7 +/- 6.9 mM (n = 14) with imax of 3.5 +/- 1.2 microA/microF. For [Ca]i transported by the exchange, a Km[Ca]i of 0.60 +/- 0.24 microM (n = 8) with an imax of 3.0 +/- 0.54 microA/microF was obtained by measuring Nao-Cai exchange current. These values are apparently different from the values for the external binding site which have been reported previously. Whether Na and Ca compete for the external binding site, and if so, how it affects the binding constants was then investigated. Outward Nai-Cao exchange current became larger by reducing [Na]o. The double reciprocal plot of the current magnitude and [Ca]o at different [Na]o revealed a competitive interaction between Na and Ca. In the absence of competitor [Na]o, an apparent Km[Ca]o of 0.14 mM was obtained. When comparing internal and external Km values, the external value is markedly larger than the internal one and thus we conclude that binding sites of the Na-Ca exchange molecule are at least apparently asymmetrical between the inside and outside of the membrane.

A-23187 evoked transmitter release from rabbit pulmonary artery and its inhibition by reactivation of sodium-pump

1. The spontaneous [3H]-release has been measured from the isolated main pulmonary artery of the rabbit preloaded with [3H]noradrenaline in the presence of uptake blockers (cocaine, 3 x 10(-5) M; corticosterone, 5 x 10(-5) M). 2. The Ca-ionophore A-23187 (3 x 10(-7)-3 x 10(-5) M) increased the outflow of [3H] by a concentration dependent manner. 3. Inhibition of Na+-pump by removal of K+ from the external medium also increased the release of labelled noradrenaline. 4. In the absence of external K+, the applied A-23187 (3 x 10(-6) M; EC50) further increased the release of [3H]. 5. Reactivation of Na+-pump by readmission of K+ (5.9 mM) to the external medium abolished the [3H]-release which had previously been increased in "K+-free" solution. 6. The reactivated Na+-pump significantly inhibited the transmitter releasing action of A-23187. 7. This latter was antagonized by an increase of external Ca2+ (7.5 mM). 8. It is concluded that the reactivated Na+-pump caused re-establishment of Na+-gradient is capable to counteract the Ca-ionophore facilitated Ca2+-influx and release from internal stores, which can be antagonized by excess Ca2+.

Effect of tetrodotoxin, lidocaine, and quinidine on the transient inward current of sheep Purkinje fibres

The effect of tetrodotoxin (TTX), lidocaine, and quinidine on the transient inward current (TI) was studied in voltage-clamped sheep cardiac Purkinje fibres. The TI was induced by elevation of extracellular Ca or addition of strophanthidin. Reduction of external Na had a biphasic effect on the steady state TI magnitude; a moderate (less than 50%) reduction of external Na had an enhancing effect on the TI; a further decrease of extracellular Na was accompanied by a decline of TI amplitude. The TI could not be induced in Na-free medium (external Ca less than or equal to 9.0 mM). TTX, lidocaine, and quinidine reduced the magnitude of the TI in a dose-dependent way. The blocking effect of these agents could be compensated for by a moderate (less than 50%) reduction of external Na or an elevation of extracellular Ca. It is suggested that the blocking effect of TTX, lidocaine, and quinidine on the TI is due to a reduction of intracellular Na, which causes a decay of intracellular Ca via the Na-Ca exchange mechanism.

The effect of ions on sodium-calcium exchange in salamander rods

1. The influence of external cations on the rate at which a Ca2+ load was eliminated in exchange for external Na+ was studied by measuring the inward current associated with Na+-Ca2+ exchange in salamander rods. 2. In Ringer solution the exchange current saturated at a well-defined level of about 20 pA at 20 degrees C. 3. The saturation level of exchange current, j(sat), was increased by lowering the external concentrations of H+, Ca2+, Mg2+ and K+; it was decreased by raising the external concentration of these ions or by lowering [Na+]O. 4. J(sat) varied approximately as [Na+]O2.4 between 35 and 110 mM-Na+. 5. The inhibitory constants for external Ca2+ and Mg2+ were about 1 and 4 mM, respectively. 6. An acid pH decreased j(sat) and an alkaline one increased it; the shape of the relation between current and pH suggests that one inhibitory proton combines between pH 8 and 10 and a pair combine between pH 6 and 7. 7. Removing K+, Mg2+, and Ca2+, and increasing the pH from 7.5 to 10 increased the measured exchange current from 20 to ca. 100 pA. 8. The integral of the Na+-Ca2+ exchange current varied with the Ca2+ load but was largely independent of external ionic changes in spite of large changes in j(sat). The apparent Na+-Ca2+ exchange ratio remained at a little under 3 over a wide range of conditions. 9. The constancy of the integral of the exchange current was brought about by reciprocal variations of the amplitude and duration of the current transient. Records in different solutions could usually be matched by scaling amplitude and time by reciprocal factors. 10. Increasing Nai+ by allowing large light-sensitive currents to flow in low-Ca2+ solutions affected the Na+-Ca2+ exchange transient in a different way from lowering [Na+]o or raising [Ca2+]o, etc. In an Na+-rich rod there was little reduction in j(sat) but the response was prolonged and larger Ca2+ loads were needed to reach saturation. Analysis in terms of a simple model indicated that a substantial Na+ load might reduce the apparent affinity of the internal pumping sites for Ca2+ by a factor of 10. 11. An attempt is made to relate these findings to a model of Na+-Ca2+ exchange.

Na/Ca exchange in barnacle muscle cells has a stoichiometry of 3 Na+/1 Ca2+

The portions of the 45Ca influx and 22Na efflux that were activated by physiological concentrations of intracellular free Ca2+, [Ca2+]i, were studied in internally perfused single giant barnacle muscle cells. Since both fluxes were activated by intracellular Ca2+ (Cai) and the Ca influx was dependent on internal Na+ (Nai), the fluxes appear to be coupled (Na/Ca exchange). Tracer Ca/Ca and Na/Na exchanges were eliminated by employing tris(hydroxymethyl)aminomethane (Tris) as the predominant external cation. Under these circumstances, the ratio of the external Ca2+ (Cao)-dependent, Cai-activated Na+ efflux to the Nai-dependent, Cai-activated Ca influx was 3.1-3.2 Na+/1 Ca2+, when the intracellular Na+ concentration, [Na+]i was either 30 or 46 mM. This is the first direct measurement of the Na/Ca exchange stoichiometry. In many types of cells, the Na/Ca exchange system appears to operate in parallel with a plasma membrane ATP-driven Ca pump that has a lower capacity (maximum velocity), but higher affinity for Ca2+ than the Na/Ca exchanger. The data on the stoichiometry and activation by internal Ca2+ imply that the turnover of the Na/Ca exchanger is modulated during periods of cell activity. When the cells are depolarized, the Na/Ca exchange system is activated by the rising [Ca2+]i, and Ca2+ entry via the exchanger is promoted. Then, at repolarization, Ca2+ exits rapidly, primarily via the exchanger. However, in resting cells, with a low [Ca2+]i, much (but not all) of the Ca2+ efflux is probably mediated by the ATP-driven Ca pump.

Identification of sodium-calcium exchange current in single ventricular cells of guinea-pig

1. The Na-Ca exchange current was investigated in single ventricular cells from guinea-pig hearts by combining the techniques of whole-cell voltage clamp and intracellular perfusion. 2. The membrane conductance was minimized by blocking Ca and K channels as well as the Na-K pump. Under these conditions, when Ca2+ was loaded internally by a pipette solution containing 430 nM-Ca2+, changing the Li+-rich external solution to a Na+-rich one induced a significant inward current. Applying external Na+ in the absence of internal Ca2+ did not appreciably change the current. 3. In contrast, perfusing 1 mM-external Ca2+ in the presence of internal Na+ which was loaded by a 20 mM-Na+ pipette solution, induced a marked outward current. Ca2+ superfusion in the absence of internal Na+ caused only a small current change. 4. The current-voltage relation of external-Ca2+- and external-Na+-induced current showed almost exponential voltage dependence as given by the equation i = a exp (rEF/RT), where a is a scaling factor that determines the magnitude of the current and r is a partition parameter used in the rate theory and represents the position of the energy barrier in the electrical field, which indicates the steepness of the voltage dependence of the current. E, F, R and T have their usual meanings. The value of a was 1-2 microA/microF and r about 0.35 for the Ca2+-induced outward current. At very positive or negative potentials, the current magnitude became smaller than expected from an exponential relation. 5. The current was blocked by heavy metal cations, such as La3+, Cd2+, Mn2+ and Ni2+ and partially blocked by amiloride and D600. 6. The temperature coefficient (Q10) value of the Ca2+-induced outward current was 3.6 +/- 0.4 (n = 4) at 0 mV and 4.0 +/- 0.9 at 50 mV in the range between 21 and 36 degrees C. 7. The outward current magnitude showed a sigmoidal dependence upon the external Ca2+ concentration with a half-maximum concentration, K1/2 of 1.38 mM and a Hill coefficient of 0.9 +/- 0.2 (n = 5). 8. Sr2+ could replace Ca2+ with K1/2 of 7 mM. Mg2+ and Ba2+, however, did not replace Ca2+. 9. The inward current component also showed a sigmoidal external Na+ dependence with K1/2 of 87.5 +/- 10.7 mM and a Hill coefficient of 2.9 +/- 0.4 (n = 6). 10. The reversal potential of the current was obtained near the values expected for 3 Na+:1 Ca2+ exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage dependence of sodium-calcium exchange: predictions from kinetic models

Voltage effects on the Na-Ca exchange system are analyzed on the basis of two kinetic models, a "consecutive" and a "simultaneous" reaction scheme. The voltage dependence of a given rate constant is directly related to the amount of charge which is translocated in the corresponding reaction step. Charge translocation may result from movement of an ion along the transport pathway, from displacement of charged ligand groups of the ion-binding site, or from reorientation of polar residues of the protein in the course of a conformational transition. The voltage dependence of ion fluxes is described by a set of coefficients reflecting the dielectric distances over which charge is translocated in the individual reaction steps. Depending on the charge of the ligand system and on the values of the dielectric coefficients, the flux-voltage curve can assume a variety of different shapes. When part of the transmembrane voltage drops between aqueous solution and binding site, the equilibrium constant of ion binding becomes a function of membrane potential. By studying the voltage dependence of ion fluxes in a wide range of sodium and calcium concentrations, detailed information on the microscopic properties of the transport system may be obtained.

Influence of membrane potential on calcium efflux from giant axons of Loligo

Experiments are described in which Ca efflux is monitored in axons under voltage clamp. As Ca efflux consists of more than one component, conditions were sought where one component predominates. Thus external Na-dependent Ca efflux can be studied in relative isolation either at pH 9.0 or in fully poisoned axons immersed in Ca-free media; external Ca-dependent Ca efflux can be studied in fully poisoned axons immersed in Na-free media and the Na-independent, energy requiring, pump is best examined in Na and Ca-free sea waters. Both in unpoisoned axons at pH 9.0 and fully poisoned axons at pH 7.8, the external Na-dependent Ca efflux is activated by hyperpolarization and inhibited by depolarization. Depolarizations achieved either electrically or by exposure to high K are roughly comparable and the inhibition brought about by high K can largely be removed by electrical hyperpolarization to the initial resting potential. In both Na sea waters and choline sea waters containing 100 mM-Na, Ca efflux is increased e-fold over approximately 50 mV. In choline sea water, external Ca-dependent Ca efflux from fully poisoned axons is unaffected by voltage over the range -80 to -30 mV. But addition of K or Li activates the flux and this activation is increased by hyperpolarization and decreased by depolarization, suggesting that the activating cation may also be transported into the axon. The Na-independent, energy-requiring, flux is inhibited by electrical hyperpolarization and stimulated by electrical depolarization. External K also stimulates the flux and part of this stimulation can be removed by electrical hyperpolarization. These data show that the energy-dependent pump is sensitive to membrane potential in the physiological range and suggest that it may be an electrogenic process. The finding that voltage affects the energy-dependent (uncoupled) pump and external Na-dependent fluxes in opposite directions may help explain why the total Ca efflux from intact axons responds to potential in a very variable manner.

Patch-clamp recordings of the light-sensitive dark noise in retinal rods from the lizard and frog

In cell-attached recordings from rods in the intact lizard retina, light decreased a standing inward membrane current with a reversal potential approximately 60 mV more positive than the resting potential. The peak amplitude of saturating responses depended upon the area of recorded membrane and varied from cell to cell over approximately 100-fold range. Small patches of membrane gave variable responses to identical moderately intense flashes. Whole-cell voltage-clamp recordings were obtained on isolated frog rods with intact ellipsoids. Peak whole-cell photocurrent was related to flash intensity by a Michaelis equation with saturating response amplitudes ranging up to 30 pA in 0.1 mM-Ca2+ Ringer solution. In darkness the steady-state current-voltage relation, determined with whole-cell voltage clamp, showed outward rectification. Photocurrent had nearly constant amplitude between -80 and -10 mV, a mean reversal potential of +8 mV and recovered from flashes more slowly at positive holding potentials. Although it was not possible to resolve light-sensitive single-channel current events, power spectral analysis revealed both low- and high-frequency components of the light-sensitive noise in both cell-attached and whole-cell recordings. The low-frequency component was described by the product of two Lorentzians using time constants derived from the kinetics of the dim flash response. The high-frequency component of the light-sensitive noise was described by a single Lorentzian with a half-power frequency of 62 Hz in lizard and 212 Hz in frog. The half-power frequency was not appreciably affected by steady illumination. The Lorentzian nature of the noise suggests that the light-sensitive channel is a pore rather than a shuttle-type carrier. In cell-attached recordings the high-frequency component declined monotonically with increasing light intensity, suggesting that less than one-half of the channels are open in darkness. Furthermore, the ratio of the variance of the high-frequency noise to the mean photocurrent was independent of light intensity. Changing external Ca2+ from 0.1 to 0.5 mM reduced the ratio from 19.7 to 9.0 fA without a significant effect on the cut-off frequency of the noise. The results support the conclusion that the light-sensitive pore is opened by an internal transmitter that acts as an agonist and that both open and closed states of the pore may be blocked by external Ca2+. The conductance of the light-sensitive pore in the absence of external Ca2+ is estimated to be 1.25-2 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

A minimum mechanism for Na+-Ca++ exchange: net and unidirectional Ca++ fluxes as functions of ion composition and membrane potential

Both simultaneous and consecutive mechanisms for Na+-Ca++ exchange are formulated and the associated systems of steady-state equations are solved numerically, and the net and unidirectional Ca++ fluxes computed for a variety of ionic and electrical boundary conditions. A simultaneous mechanism is shown to be consistent with a broad range of experimental data from the squid giant axon, cardiac muscle and isolated sarcolemmal vesicles. In this mechanism, random binding of three Na+ ions and one Ca++ on apposing sides of a membrane are required before a conformational change can occur, translocating the binding sites to the opposite sides of the membranes. A similar (return) translocation step is also permitted if all the sites are empty. None of the other states of binding can undergo such translocating conformational changes. The resulting reaction scheme has 22 reaction steps involving 16 ion-binding intermediates. The voltage dependence of the equilibrium constant for the overall reaction, required by the 3:1 Na+: Ca++ stoichiometry was obtained by multiplying and dividing, respectively, the forward and reverse rate constants of one of the translocational steps by exp(-FV/2RT). With reasonable values for the membrane density of the enzyme (approximately 120 sites micron 2) and an upper limit for the rate constants of both translocational steps of 10(5) . sec-1, satisfactory behavior was obtainable with identical binding constants for Ca++ on the two sides of the membrane (10(6) M-1), similar symmetry also being assumed for the Na+ binding constant (12 to 60 M-1). Introduction of order into the ion-binding process eliminates behavior that is consistent with experimental findings.

Na-Ca exchange: stoichiometry and electrogenicity

This review discusses the evidence concerning the stoichiometry of Na-Ca exchange. In particular we consider whether the Na-Ca exchange has been shown to transport more than two Na+ ions per Ca2+ ion and therefore whether it generates an electric current. The first part of this review discusses both direct and indirect evidence concerning the stoichiometry of the exchange and its possible voltage dependence. We find that, although there is some evidence suggesting that more than two Na+ ions may exchange for each Ca2+ ion, most of the available evidence is equivocal and cannot fix the stoichiometry precisely. Furthermore, using a simple and explicit circulating carrier model for the Na-Ca exchange, we show that the effect of membrane potential on the Na-Ca exchange may be considerably more complicated than is generally believed. In particular we find that both electrogenic and electroneutral exchanges will be affected by membrane potential. We therefore conclude that the demonstration of the voltage dependence of the Na-Ca exchange does not necessarily imply that it is electrogenic. Additionally, this analysis shows that, apart from a restricted range near thermodynamic equilibrium, it is impossible to predict either the magnitude or the direction of the effects of membrane potential on the exchange. In the second part of the review we consider whether any known membrane currents may be attributed to Na-Ca exchange. We show, in contrast to previous suggestions, that the Na-Ca exchange can theoretically produce a current that appears to be activated by intracellular Ca and that has a reversal potential. However, the experimental demonstration that a given current is produced by Na-Ca exchange is hampered by the existence of other Ca- and Na-dependent currents. In conclusion, we feel that there is no evidence that allows any particular membrane current to be unambiguously identified with the Na-Ca exchange.

The Na,K-ATPase hypothesis for manic-depression. II. The mechanism of action of lithium

A model as to how lithium may work in the treatment and prevention of manic-depression is presented. Lithium accumulates intracellularly, and accumulates preferentially in more active neurons. Intracellular accumulation of lithium displaces intracellular sodium, which, in turn, decreases intracellular calcium. A decrease of intracellular calcium normalizes neuron activity in both mania and depression. This model is supported by the majority of clinical and experimental data.

The Na,K-ATPase hypothesis for manic-depression. I. General considerations

An hypothesis is presented to explain and integrate experimental and clinical observations on manic-depressive (bipolar or biphasic) psychosis. The model is based on alterations in the activity of the sodium, potassium-activated adenosine triphosphatase (Na, K-ATPase) pump. A reduction in the activity of the Na,K-ATPase can be responsible for both phases of the disorder.

The control of tonic tension by membrane potential and intracellular sodium activity in the sheep cardiac Purkinje fibre

Intracellular Na activity (aiNa) was measured with recessed-tip, Na-selective micro-electrodes in voltage-clamped sheep cardiac Purkinje fibres. Tension was measured simultaneously. aiNa was increased reversibly either by exposing the preparation to K-free, Rb-free solution of by adding the cardioactive steroid strophanthidin. An increase of aiNa produced an increase of tonic tension which was larger at depolarized membrane potentials. At sufficiently negative membrane potentials, changes of aiNa (over the range 6-30 mM) had no effect on tonic tension. Therefore, both an increase of aiNa and a depolarization are required to increase tonic tension. It is concluded that either a low level of aiNa or a large negative membrane potential is sufficient to maintain a low intracellular Ca concentration. Tonic tension was measured as a function of aiNa. At a given membrane potential the relationship can be described empirically by an equation of the form: tonic tension = b(aiNa)y, where y is a constant and b depends on membrane potential. In five experiments y was found to be 3.7 +/- 0.7 (mean +/- S.E.M.) over a range of potentials from -60 to -10 mV. Tonic tension was measured as a function of membrane potential. At a given aiNa the relationship can be described approximately as: tonic tension = k exp (aV), where a is a constant and k depends on aiNa. In five experiments a was found to be 0.06 +/- 0.01 mV-1 (mean +/- S.E.M.). A depolarization of 10 mV increases tonic tension by the same amount as does an increase of aiNa that is equivalent to a 3.7 mV change of the Na equilibrium potential, ENa. Hence ENa is nearly 3 times more effective than membrane potential in controlling tonic tension. During a prolonged depolarization (several minutes) the initial increase of tonic tension decays gradually. This is associated with a fall of aiNa. The relationship between tonic tension and aiNa is similar to that seen when aiNa is increased by inhibiting the Na pump. It is concluded that the fall of aiNa is responsible for the decay of tonic tension. The changes of tonic tension reported in this paper are consistent with the effects of aiNa and membrane potential on a voltage-dependent Na-Ca exchange. The possibility that a voltage-dependent Ca channel contributes to tonic tension is also discussed.

Calcium efflux from the rat neurohypophysis

1. Calcium efflux from isolated rat neurophypophyses has been studied. Curve fitting of the wash-out curves suggests three phases with t((1/2)) of ca. 3, 15 and 130 min.2. The slow component of the (45)Ca efflux is attributed to efflux of intracellular Ca. On the basis of the temperature sensitivity of the Ca efflux, the activation energy has been calculated to be approximately 12,000 cal/mole, corresponding to a Q(10) of ca. 2.0.3. Ca efflux decreased by approximately 32% when external Na was replaced by choline. Li(o), in the presence or absence of Ca(o), was as effective as Na(o) in stimulating the Ca efflux.4. The curve relating Ca efflux to [Na](o) or [Li](o) is sigmoid and suggests that at least two Na (or Li) ions are necessary to activate the efflux of each Ca ion. Ca(o) does not modify the absolute Na-dependent Ca efflux but decreases the affinity for Na of the site involved in Ca extrusion.5. Removal of Ca(o) decreased the Ca efflux by ca. 44% in Na-free media. The apparent affinity for Ca(o) of the Ca(o)-activated Ca efflux (K(m) (Cao) = 20 muM) is greatly decreased by the presence of 150 mM-Na (K(m) (Cao) = 0.8 mM).6. Lanthanum decreased the total Ca efflux by ca. 60% and totally abolished the Na(o)-activated and Ca(o)-activated Ca efflux.7. Vanadate reduced the Ca efflux remaining in Na-, Ca-free saline by 73%.8. Elevation of Na(i) with ouabain did not modify the rate of loss of (45)Ca.9. Increased concentration of K(o) stimulated transiently the (45)Ca loss. The time course of this increase depends on the Ca(o) concentration ([Ca](o)).10. Cyanide or CCCP (carbonyl cyanide m-chlorophenylhydrazone) increased transiently the Ca efflux. The increase induced by cyanide could only be observed when the neural lobes had been over-loaded with (45)Ca.11. Membrane destruction induced by high temperature eliminated the effect of [Na](o) and [Ca](o) on (45)Ca efflux.12. In 150 mM-Na-containing saline, half-maximum activation of (45)Ca uptake occurs in the 0.2-0.4 mM [Ca](o) range.13. The Ca efflux from isolated pituicytes was not affected by removal of Na(o).14. In conclusion we show that Ca efflux from neurosecretory nerve terminals can be subdivided into three components of approximately the same magnitude, one which is activated by Na(o), another by Ca(o) and a third component which is independent of Na(o) and Ca(o).

Sodium-dependent and calcium-dependent calcium transport by rat brain microsomes

Microsomal vesicles prepared from rat brain contain a Na+-Ca2+ exchange transport system capable of accumulating Ca2+ in a time- and temperature-dependent manner. The Ca2+ accumulated by these vesicles was released by the Ca2+ ionophore A23187 but not by EGTA. The Km value for Ca2+ uptake was 23 microM with a maximal velocity of 21 nmol Ca2+/mg per min. Ca2+ uptake was significantly inhibited by La3+, Sr2+, Mn2+ and Ba2+ and to a lesser extent by Mg2+. 45Ca2+ accumulated by Na+-dependent uptake could be released by 40Ca2+, indicating the presence of a Ca2+-Ca2+ exchange activity in the microsomes. Ca2+-Ca2+ exchange was stimulated in Li+- and K+-containing media as compared to choline+ media. Microsomes also catalyzed ATP-dependent Ca2+ uptake (in the absence of Na+ gradient). The Ca2+ sequestered by this mechanism could be released by extravesicular Na+, indicating that both the ATP-dependent and the Na+-dependent Ca2+ uptake systems are present in the same membrane. The microsomal preparation used did not contain measurable amounts of succinate dehydrogenase activity or oligomycin-azide-dinitrophenol sensitive ATP-dependent Ca2+ uptake. Thus, the Ca2+ accumulation observed was not due to contaminating mitochondria. The preparation was enriched for 5'-nucleotidase and (Na+ + K+)-ATPase (plasma membrane markers) as well as antimycin A-resistant NADPH-dependent cytochrome c reductase activity (an endoplasmic reticulum marker).

Potential and tension changes induced by sodium removal in dog Purkinje fibres: role of an electrogenic sodium-calcium exchange

1. Isolated dog Purkinje fibres were bathed in K-free media or in the presence of ouabain 10(-4) M in order to depress the electrogenic sodium pump activity. Membrane potential and mechanical tension were recorded in the presence of normal external sodium concentration and during lowering or removal of external Na. 2. Lowering or removal of external Na (Na being replaced by choline, Tris, sucrose or Li) induced a hyperpolarization and a contracture which reached a maximum after 1 or 2 min and then decreased progressively. Using Tris, Em increased from -40 +/- 3 to -72 +/- 10 mV (n = 39). The Na-free contracture and hyperpolarization did not occur in the absence of Na pump depression. 3. Tetrodotoxin (1.2 x 10(-5)M), Mn (4 mM), verapamil (1-4 x 10(-5) M) tetraethylammonium (5 mM), 4-aminopyridine (5 mM) and Cs (20 mM, in the presence of ouabain) did not alter the Na-free contracture and hyperpolarization. On the other hand Mn (20 mM), acid media (external pH less than 6.0) and low temperatures depressed or suppressed both the hyperpolarization and contracture. Lanthanum (0.4 mM) did not suppress the hyperpolarization and the contracture. On the contrary the Na-free contracture was generally increased in the presence of La. 4. Caffeine (10 mM) induced strong contractures with no changes in Em, thus demonstrating the possibility for the Purkinje fibers of developing contractures without concomitant hyperpolarizations. 5. It can be concluded that the Na-free contracture and hyperpolarization are not due to changes in passive conductances but are related to the functioning of an electrogenic Na-Ca exchange mechanism which carries inwardly 1 Ca and outwardly 3 or more Na.

Contribution of Na/Ca transport to the resting membrane potential

Relations are derived that describe the combined effects of electrodiffusion, the Na/K pump, and Na/Ca transport by carrier on the resting membrane potential. Equations are derived that apply to both steady-state and non-steady-state conditions. Some example calculations from the equations are plotted at different permeability coefficient ratios, PK:PCa:PNa. The equations predict a depolarizing action of Na/Ca transport when more than two Na ions per Ca ion are transported by the carrier. For all permeability ratios examined, a steady state for Ca ions is achieved with at most a few millivolts of depolarization.

Sodium-calcium exchange activity generates a current in cardiac membrane vesicles

Sarcolemmal membrane vesicles isolated from canine ventricular tissue accumulate calcium through the sodium-calcium exchange system when an outwardly directed sodium gradient is generated across the vesicle membrane. Moreover, calcium uptake under these conditions is accompanied by the transient accumulation of the lipophilic cation tetraphenylphosphonium. Since the distribution of tetraphenylphosphonium across biological membranes reflects the magnitude and direction of transmembrane potential differences and the characteristics of the transient accumulation of this cation closely resemble those of sodium-calcium exchange activity, it is concluded that a membrane potential, interior negative, is produced during calcium accumulation through the exchange system. Thus, the operation of the sodium-calcium exchange system generates a current in cardiac membrane vesicles, suggesting that three or more sodium ions exchange for each calcium ion.

Calcium movement in nerve fibres

Given the existence of a difference in electrical potential between the interior of a nerve cell and the media surrounding it, where the cytoplasm is some 70 mV negative (Hodgkin, 1958), it must be expected that any positively charged ion to which the cell membrane is permeable is more concentrated in the cell interior. For monovalent cations such as Na and divalent cations such as Ca and Mg this is not the case in the majority of the cells such as the squid giant axon. In other words, nerve cells maintain a lower intracellular concentration of these ions, as compared with their concentration in the extracellular fluid. For Mg, Ca and Na ions, this lower internal concentration must, in the steady state, be effected by some membrane based mechanism which consumes energy.

Lithium fluxes in human erythrocytes

The contribution of four transport pathways to Li+ influx and efflux in human erythrocytes was determined quantitatively, using Li+ concentrations comparable to those found in vivo when Li+ is used as treatment for manic-depressive illness. All pathways were measured simultaneously on each subject's blood sample to avoid possible temporal variations in transport parameters. We found that Li+ efflux is 75% via countertransport and 25% via a leak. The bicarbonate-sensitive pathway accounts for 30% of influx while the remaining 70% is via a leak. The Na+-K+ pump makes no significant contribution to Li+ influx or efflux under physiological conditions. Li+ efflux for a given [Li+]i is 3-5 times the Li+ influx for the same [Li+]o. However, due to interindividual variations in Na+-Li+ counter-transport, Li+ efflux but not influx varies considerably among individuals.

Ionic basis of transient inward current induced by strophanthidin in cardiac Purkinje fibres

1. Voltage clamp experiements studied the ionic basis of the strophathidin-induced transient inward current (TI) in cardiac Purkinje fibres. 2. The reversal potential of TI (Erev) was determined in the presence of various bathing solutions. Erev averaged --5 m V in the standard modified Tyrode solution (Kass, Lederer, Tsien & Weingart, 1978). Erev was displaced toward more negative potentials when the external Na concentration (NaO) was reduced by replacement of NaCl with Tris Cl, choline Cl or sucrose. 3. A sudden reduction of NaO evoked a temporary increase in TI, followed after a few minutes by a sustained diminution. The initial increase was closely paralleled by an enhanced aftercontraction and could be explained by an indirect effect of NaO on internal Ca. The subsequent fall in TI amplitude could be accounted for by the reduced driving force, E--Erev. 4. Erev was not significantly changed by replacing extracellular Cl with methyl-sulphate, or by limited variations in external Ca (2.7--16.2 mM) or external K (1--8 MM). 5. These results are consistent with an increase in membrane permeability to Na and perhaps K. 6. TI was not directly affected by TTX, which blocks excitatory Na channels, or by Cs, which inhibits inwardly rectifying K channels. TI may be distinguished from the slow inward current by its kinetic, pharmacological and ionic properties. 7. TI might be carried by a pre-existing ionic pathway such as the 'leak' channel which provides inward current underlying normal pace-maker depolarization. Another possibility is that TI reflects Ca extrusion by an electrogenic Ca--Na exchange.

Allosteric cooperativity during intestinal cotransport of sodium and chloride in freshwater prawns

Coupled influxes of sodium and chloride across the mucosal border of a freshwater prawn intestine were sigmoidal functions of luminal ion concentrations, indicative of a cooperative allosteric transport process. This process had a higher affinity for Cl (KCl = 94 mM) than for Na (KNa = 155 mM), maximally transported twice as much cation as anion (JNa max = 1.6; JCl max = 0.75 mumol-cm-2-h-1), and exhibited identical Hill interaction indices for both ions nNa = 3.4; nCl = 3.5). The suggestion is made that this cooperative carrier mechanism may be regulatory, maintaining relatively constant luminal ion concentrations which, in turn may facilitate ion-dependent absorption of non-electrolytes.

Effects of internal and external cations and of ATP on sodium-calcium and calcium-calcium exchange in squid axons

Calcium-45 efflux was measured in squid axons whose internal solute concentration was controlled by internal dialysis. Most of the Ca efflux requires either external Na (Na-Ca exchange) or external Ca plus in alkali metal ion (Ca-Ca exchange; cf. Blaustein & Russell, 1975). Both Na-Ca and Ca-Ca exchange are apparently mediated by a single mechanism because both are inhibited by Sr and Mn, and because addition of Na to an external medium optimal for Ca-Ca exchange inhibits Ca efflux. The transport involves simultaneous (as opposed to sequential) ion counterflow because the fractional saturation by internal Ca (Cai) does not affect the external Na (Nao) activation kinetics; also, Nao promotes Ca efflux whether or not an alkali metal ion is present inside, whereas Ca-Ca exchange requires alkali metal ions both internally and externally (i.e., internal and external sites must be appropriately loaded simultaneously). ATP increases the affinity of the transport mechanism for both Cai and Nao, but it does not affect the maximal transport rate at saturating [Ca2+]i and [Na+]o; this suggest that ATP may be acting as a catalyst of modulator, and not as an energy source. Hill plots of the Nao activation data yield slopes congruent to 3 for both ATP-depleted and ATP-fueled axons, compatible with a 3 Na+-for-1 Ca2+ exchange. With this stoichiometry, the Na electrochemical gradient alone could provide sufficient energy to maintain ionized [Ca2+]i in the physiological range (about 10(-7) M).